The invention relates to a fluid friction clutch, especially for a radiator fan of an internal combustion engine.
A fluid friction clutch for the radiator fan of an internal combustion engine is known from U.S. Pat. No. 4,086,989. The fluid friction clutch has a clutch cover provided with fan blades, which is rotatably mounted on an input shaft driven in rotation by the internal combustion engine. The cover is divided by a partition axially into a reservoir for shear fluid and a working chamber. In the working chamber there is arranged a rotor of disc form connected with the input shaft, the external circumference of which rotor forms with the cover an annular chamber or gap. The side faces of the rotor with side faces of the cover and the partition form shear gaps which, when they are filled with the viscous shear fluid, couple the rotor with the cover. The clutch is actuated by a valve which, for example temperature-dependently, controls the inflow of shear fluid from the reservoir into the working chamber. At high temperatures the valve opens, so that shear fluid can transfer from the reservoir into the working chamber. At low temperatures the valve is closed and a dynamically working pump device pumps the shear fluid from the working chamber back into the reservoir. The pump device comprises a baffle body held on the cover and extending into the annular chamber formed between the external circumference of the rotor and the internal circumference of the cover. Behind the baffle body, in the drive direction of rotation of the rotor, a shear fluid transfer opening is arranged in the partition in the region of the annular chamber. Since the cover always rotates at a lower rotation rate than the rotor by reason of the slip between cover and rotor, the shear fluid builds up in the annular chamber behind the baffle body and is forced through the transfer opening into the reservoir.
The pump device has to pump the shear fluid away out of the annular chamber against the pressure of the shear fluid in the reservoir. The pressure in the reservoir increases with increasing fluid level or fluid quantity in the reservoir, so that operational situations can occur in which a residual quantity of shear fluid can no longer be pumped away out of the annular chamber. The residual quantity of shear fluid in the working chamber increases the idling rotation rate of the disengaged clutch, which is undesired especially at low temperatures or in a cold-starting of the internal combustion engine. The invention is directed to improve the pump effect of a fluid friction clutch, especially in the case of a small residual volume of shear fluid in the working chamber.